1. Field of the Invention
The present invention relates to an electrolytic capacitor having an excellent heat-resisting property, a high ripple current property, and a long lifetime, and to an electrolytic solution used for the electrolytic capacitor.
2. Background Art
In recent years, as electronic equipment has a smaller size and higher performance, an electrolytic capacitor used in a power circuit of electronic equipment is demanded to have high reliability such as high a heat-resisting property, a high ripple current property, and a long lifetime. In particular, an electrolytic solution that is one of main components of an electrolytic capacitor is demanded to have excellent properties when it is used in a high temperature environment.
FIG. 3 is a sectional view showing a radial-lead type aluminum electrolytic capacitor that is one of conventional electrolytic capacitors. This aluminum electrolytic capacitor includes capacitor element 102, cylindrical case 103 having a bottom, sealing material 104, and an electrolytic solution (not shown).
Capacitor element 102 is formed by winding an anode foil and a cathode foil with a separator interposed therebetween (not shown). Lead wires 101 for external lead-out are connected to the anode foil and the cathode foil, respectively. Case 103 contains capacitor element 102 impregnated with the electrolytic solution. Sealing material 104 is provided with through-holes 104A through which lead wires 101 are inserted. Sealing material 104 seals an opening of case 103.
Furthermore, the electrolytic solution includes a carboxylic acid having a carboxyl group and an alkyl group as a substituent on each terminal carbon of a main chain as shown in general formula (A).

A general electrolytic solution may include a compound having a carboxyl group as an electrolyte. In such a case, this carboxyl group tends to undergo an esterification reaction with alcohol such as ethylene glycol as a solvent in a high temperature environment. The reaction product subsequently undergoes an amidation reaction with ammonia, amine, or the like, contained in a general electrolytic solution. Due to the thus produced compound, the electric conductivity of the electrolytic solution is considerably reduced over time. As a result, the equivalent series resistance (hereinafter, referred to as “ESR”) of the electrolytic capacitor is remarkably increased, and therefore the ripple current property cannot be improved and the lifetime cannot be extended.
On the contrary, a carboxylic acid having a structure represented by general formula (A) has a carboxyl group and alkyl groups (R1 and R2) as a substituent on both terminal carbons of the main chain. Therefore, in an electrolytic solution including such a carboxylic acid as an electrolyte, the alkyl group is adjacent to the carboxyl group to serve as a steric hindrance, thus inhibiting the reaction between the carboxyl group and alcohol such as ethylene glycol as a solvent. Therefore, the above-mentioned esterification reaction and amidation reaction are suppressed. Consequently, the reduction over time in the electric conductivity of the electrolytic solution can be prevented in a high temperature environment. As a result, the increase in the ESR of an electrolytic capacitor is suppressed, and an electrolytic capacitor having a high heat-resisting property, a high ripple current property, and a long lifetime can be provided. Such a conventional technology is disclosed in, for example, Japanese Patent Application Unexamined Publication No. 2005-5336.
However, even when an electrolytic solution including a carboxylic acid having the structure represented by general formula (A) as an electrolyte is applied, it is not possible to sufficiently suppress the increase over time in the ESR in a high temperature environment. In particular, the change in the ESR in a low frequency region of about 50 to 120 Hz is still large, so that an electrolytic capacitor having a high heat-resisting property, a high ripple current property and a long lifetime cannot be realized.